Generally, in a single phase induction motor, a main coil and a sub coil are wound in a stator at 90° in space, and a power voltage is applied to the main coil directly and to the sub coil through a capacitor and a switch. It is because a rotor is not started merely by the main coil, although a voltage is applied thereto. Therefore, a rotor field is produced in the stator through a starting device such as the sub coil to get the rotor started.
The starting devices are classified into a split phase type, a shading coil type, a capacitor start type and a repulsion start type according to kinds.
As one example of the single phase induction motor having the starting device, a capacitor start type single phase induction motor is illustrated in FIGS. 1 and 2.
FIG. 1 illustrates a stator and a rotor of a normal single phase induction motor, and FIG. 2 illustrates a simplified circuit of the rotor and stator coils.
When only a main coil 12 is wound in a stator 10, only an alternating magnetic field is produced in the stator 10, so that a rotor 20 is not started. Accordingly, a sub coil 14 is wound in the stator 10 to produce a rotating magnetic field, such that the rotor 20 is started and rotated in a certain direction by the rotating magnetic field. That is, a starting torque is generated via the rotating magnetic field.
Here, a capacitor 15 delays a phase of a current applied to the sub coil 14 to generate the starting torque through mutual operation with the main coil 12. Although power is not applied to the sub coil 14, the rotor 20 continues to rotate once started, as far as there is no change in load. Therefore, it is not necessary to apply power to the sub coil 14 over a certain revolution number after starting. However, if the load is variable, since the starting torque is necessary, it is preferable to supply power to the sub coil 14 through the capacitor 15 all the time.
In the case of a three phase induction motor, although only a main coil is wound in a stator, a rotating field is produced, so that it is not necessary to wind a sub coil in the stator. That is, the three phase induction motor does not need a special starting device.
The single phase induction motor has superior price competitiveness because it does not require an inverter construction unlike a Brushless DC (BLDC) motor or a reluctance motor and can be started by single phase commercial power.
The normal single phase induction motor will be explained in detail with reference to FIGS. 1 and 2.
The stator 10 has a hollow structure, and consists of a plurality of teeth 11 placed along an inner circumference at intervals of a certain angle to protrude to an inner radius direction, and the main coil 12 wound on the respective teeth 11 to have N or S polarity in primary current application.
Here, an insulator (not shown) is provided between the teeth 11 and the main coil 12 to perform an insulation function between the teeth 11 and the main coil 12 as well as a function of causing the main coil 12 to be easily wound.
In addition, the stator 10 includes the sub coil 14 wound at a certain angle in space from the main coil 12 to produce a rotating magnetic field as the current is applied. The sub coil 14 is also wound on the teeth 11 through the insulator. The main coil 12 and the sub coil 14 can be referred to as a stator coil or a coil. The coils 12 and 14 are connected to a single phase power supply, and the main coil 12 and the sub coil 14 are connected in parallel to each other. Moreover, the capacitor 15 is connected in series to the sub coil 14. Further, although not illustrated, the capacitor 15 may be selectively connected to the power supply through a switch.
The rotor 20 is normally a squirrel cage rotor. FIGS. 1 and 2 illustrate the squirrel cage rotor.
The rotor 20 is formed by stacking steel plates in which a plurality of slots 21 are formed at certain angles along the outer circumference in a certain radius position from the center. Also, the rotor 20 includes bar-shaped conductors 22 inserted into the slots 21 of a rotor core. The bar-shaped conductors 22 are normally formed of Cu or Al bars.
In addition, both ends of the squirrel cage rotor core are connected by end rings (not shown) to form electric short through the bar-shaped conductors 22. Generally, they are die-casted. That is, the bar-shaped conductors 22 and the end rings are integrally formed by Al diecasting, and the end rings are formed at upper and lower parts of the rotor core respectively.
Meanwhile, an axial hole 24 is formed in a central portion of the rotor 20. A rotation shaft (not shown) which transfers a rotation force of the rotor 20 to the outside is press-fit into the axial hole 24, and rotated integrally with the rotor 20.
In the single phase induction motor so constructed, when power is applied to the coils 12 and 14, an induced current is produced in the bar-shaped conductors 22, so that the rotor 20 is rotated by an induced torque generated thereby.